1. Field of the Invention
The present invention relates to a malfunction diagnosis device for a fuel-evaporated-gas processing device of, for example, a vehicle engine, and more specifically, to a malfunction diagnosis device for a fuel-evaporated-gas processing device having a function of concentrically detecting a malfunction of components or a device related to the control of an exhaust gas (hereinafter, referred to as "exhaust-gas-related-components").
2. Description of the Related Art
Recently, as greater attention has been paid to problems of terrestrial environment, there is a tendency that a gas exhausted from vehicles such as motor cars is more strictly regulated. Consequently, there must be provided a function for checking whether exhaust-gas-related-components are normally operating or not.
It is contemplated that the exhaust-gas-related-components include, for example, a fuel-evaporated-gas processing device for processing a fuel evaporated gas generated from a fuel tank, a fuel device for supplying fuel to an engine, a misfire detection device for monitoring whether fuel is normally burnt in an engine or not, and an oxygen (O.sub.2) sensor as a main component necessary to feed back oxygen (O.sub.2) to increase the purifying efficiency of a catalyst and the like. Although the O.sub.2 sensor is also one of the components constituting the fuel device, the O.sub.2 sensor will be described independently of the fuel device to make the description more understandable.
As a conventional malfunction diagnosis device for a fuel-evaporated-gas processing device mounted on a vehicle, there is proposed a device for making a determination of a malfunction independently of a malfunction determining function of other exhaust-gas-related-components such as, for example, the misfire detection device, the fuel device, the O.sub.2 sensor and the like (refer to, for example, Japanese Patent Laid-Open No. 2(1990)-26754).
FIG. 12 is a view showing the arrangement of a conventional malfunction diagnosis device for a fuel-evaporated-gas processing device mounted on a vehicle.
The malfunction diagnosis device includes a fuel tank 1 filled with fuel, a pressure sensor 2 for detecting the pressure in the fuel tank 1, a canister 3 containing activated charcoal as an absorbing agent for absorbing a fuel evaporated gas generated in the fuel tank 1, a solenoid valve 4 for opening and closing a vent passage (not shown) connecting the canister 3 to the outside (atmosphere), a solenoid valve 6 located in a fuel vapor supply passage 5 between the canister 3 and an intake pipe 7 of an engine 8 for supplying the fuel evaporated gas absorbed by the canister 3 to the engine 8, and an engine control unit (hereinafter referred to as an ECU) 9 for controlling the engine 8.
The malfunction diagnosis device is mounted on an exhaust pipe 11 of the engine 8 and includes an oxygen (O.sub.2) sensor 10 for detecting an air/fuel ratio of a mixture (a weight ratio of intake air sucked into the engine 8 to fuel supplied to the engine 8) and generating a corresponding output signal to the ECU 9. The ECU 9 outputs control signals to a plurality of injectors 12 provided one for each of the cylinders at the intake manifold of the engine 8 in response to an output detected by the O.sub.2 sensor 10.
The malfunction diagnosis device further includes a crankshaft sensor 13 mounted on the crankshaft of the engine 8 for outputting a signal at each predetermined angle of the crankshaft and generating a corresponding output signal to the ECU 9, and a water temperature sensor 14 for detecting the temperature of the cooling water of the engine 8 and generating a corresponding output signal to the ECU 9.
The components 2-6 and 9 constitute the fuel-evaporated-gas processing device; the components 13 and 9 constitute the misfire detection device; and the components 9, 10, 12 and 14 constitute the fuel device.
Next, the operation of the above-mentioned conventional malfunction diagnosis device will be described.
First, an operation for determining a malfunction of the fuel-evaporated-gas processing device effected by detecting the pressure in the fuel tank 1 will be described with reference to FIG. 13.
A fuel evaporated gas stored in the fuel tank 1 is absorbed by the activated charcoal in the canister 3. Although the vent passage extending from the canister 3 to the atmosphere is usually opened to the atmosphere by the solenoid valve 4, when an abnormal or excessive amount of the fuel evaporated gas is absorbed by the canister 3, the vent passage is used as an emergency passage for exhausting the fuel evaporated gas to the outside of the canister 3.
The ECU 9 monitors the operating state of the engine 8 based on the information from the sensors 2, 10, 13, 14 mounted on the respective portions of the engine 8, and when the ECU 9 recognizes that the engine is operating in such a state that a fuel evaporated gas is absorbed by the canister 3, the ECU 9 determines that it is in a fuel-evaporated-gas processing device check mode (time T.sub.0) and closes the vent passage of the canister 3 and the fuel vapor supply passage 5 by turning off the solenoid valves 4 and 6 to thereby close the entire fuel-evaporated-gas passage.
With this operation, since the fuel evaporated gas in the fuel tank 1 cannot escape to anywhere, the fuel tank 1 is filled with an increasing amount of the fuel evaporated gas and the pressure in the fuel tank 1 is increased to a certain level P.sub.0. After this state has continued for a predetermined period of time, the solenoid valve 6 is turned on to open the fuel vapor supply passage 5 (time T.sub.1) so that the fuel evaporated gas filled in the canister 3 is discharged through the canister 3 and the fuel vapor supply passage 5 to the intake pipe 7 within a predetermined time (until time T.sub.2) and the high pressure in the fuel tank 1 is decreased to a predetermined low pressure P.sub.1 accordingly.
Thereafter, the solenoid valve 6 is turned off to close the fuel vapor supply passage 5 again and a period of time tm necessary for the pressure in the fuel tank 1 to increase by a predetermined pressure P.sub.2 is measured.
Although time tm is equal to time t.sub.0 when the fuel-evaporated-gas processing device normally operates, when the fuel-evaporated-gas passage is partially damaged in the area, for example, from the fuel tank 1 to the intake pipe 7 or the solenoid valve 4 or 6 is damaged, the fuel evaporated gas leaks so that the relationship t.sub.m =t.sub.1 is established and a long time is required for the increase of the pressure in the fuel tank 1.
Consequently, the malfunction of the fuel-evaporated-gas processing device can be determined depending upon a change in the internal pressure of the fuel tank 1, i.e., whether the pressure increasing time tm is long or short.
Next, an operation for determining a malfunction of the fuel-evaporated-gas processing device effected by a change in the air fuel/ratio (A/F ratio) of the engine 8 will be described with reference to FIG. 14.
The ECU 9 determines the operating state of the engine 8 by detecting an engine rotational speed (RPM) through the crankshaft sensor 13 and an engine warming-up state through the water temperature sensor 14. When the engine operating state is such that the warming up of the engine 8 has finished and that the engine 8 is in a mode in which O.sub.2 feedback control can be effected, the ECU 9 determines that the engine is in the fuel-evaporated-gas processing device check mode (time T.sub.10), and it closes the vent passage of the canister 3 and the fuel vapor supply passage 5 by turning off the solenoid valves 4 and 6 so as to close the entire fuel-evaporated-gas passage. With this operation, the fuel evaporated gas in the fuel tank 1 cannot escape to anywhere so that the fuel tank 1 is filled with the fuel evaporated gas. After this state has continued for a predetermined period of time, the solenoid valve 6 is turned on (time T.sub.11) to discharge the fuel evaporated gas filled in the canister 3 to the engine 8 in a moment.
On the other hand, the O.sub.2 feedback control is continuously carried out in the check mode and an O.sub.2 feedback control compensation amount K.sub.FB acts to reverse an output from the O.sub.2 sensor 10 (at A/F ratio=14.7), as shown in FIG. 14, so that fuel is controlled by compensating the pulse width of a control signal supplied to the injector 12 of FIG. 12 based on the feedback control compensation amount K.sub.FB.
When the feedback control compensation amount K.sub.FB is represented by K.sub.FBU1, K.sub.FBU2, . . . at the time an output from the O.sub.2 sensor 10 is reversed from lean to rich in a fuel-evaporated-gas shut off period from the time T.sub.10 to the time T.sub.11 (both solenoid valves 4 and 6 are turned off) as well as when the feedback control compensation amount K.sub.FB is represented by K.sub.FBL1, K.sub.FBL2, . . . at the time the output from the O.sub.2 sensor 10 is reversed from rich to lean on the contrary, an average feedback control compensation amount K.sub.FBM is calculated according to the following formula. EQU K.sub.FBM =(K.sub.FBU1 +K.sub.FBL1)/2+(K.sub.FBU2 +K.sub.FBL2)/2+(1)
Thereafter, after the fuel evaporated gas is supplied to the engine 8 for a predetermined period of time from the time T.sub.11, the feedback control compensation amount K.sub.FB (K.sub.FB12) is measured (time T.sub.12) and a difference K.sub.FB between the amount K.sub.FB12 and the average feedback control compensation amount K.sub.FBM is calculated by the following formula. EQU .DELTA.K.sub.FB =K.sub.FMB -K.sub.FB12 ( 2)
When the fuel-evaporated-gas processing device normally operates, the fuel evaporated gas (mixed rich gas) filled in the canister 3 from the time T.sub.10 to the time T.sub.11 is supplied to the engine 8 after the time T.sub.11. To control the mixed gas to an A/F ratio of 14.7 by the O.sub.2 feedback control, the feedback control compensation amount .DELTA.K.sub.FB is set to a small value (compensation to a lean value) and K.sub.FB is set to a large value.
When, for example, the fuel-evaporated-gas passage from the fuel tank 1 to the engine 8 is partially damaged or the solenoid valve 4 or 6 is damaged so as to allow leakage of the fuel evaporated gas, the canister 3 is not filled with a mixed rich gas from time T.sub.10 to time T.sub.11, so that even if the solenoid valve 6 is turned on, the A/F ratio of the mixed gas supplied to the engine 8 is not made rich after the time T.sub.11. As a result, a compensation for making the mixed gas lean is not carried out by an O.sub.2 feedback control coefficient and K.sub.FB is set to a small value as compared with the case where the fuel-evaporated-gas processing device normally operates.
As described above, a malfunction of the fuel-evaporated-gas processing device can be determined by monitoring an amount of change in the air/fuel ratio of a mixture supplied to the engine 8, i.e., .DELTA.K.sub.FB.
Next, operation for determining a malfunction of the misfire detection device will be described with reference to FIG. 15.
The ECU 9 detects the RPM of the engine 8 by measuring a signal cycle from the output signal of the crankshaft sensor 13. When misfire takes place in the engine 8 at time T.sub.1 in FIG. 15, torque is not produced in a cylinder which is misfiring, so that the RPM of the crankshaft of the engine 8 decreases, and as a result, the cycle of a signal output from the crankshaft sensor 13 is extended. Thus, when the misfire occurred at time T1, the cycle of the crankshaft sensor signal is extended to T.sub.B1 at time T.sub.2. The occurrence of the misfire is detected by an extended length of the signal cycle T.sub.B1 beyond a predetermined misfire determination level T.sub.B2, and thus the malfunction of a component of an ignition system can be determined.
Next, an operation for determining a malfunction of the O.sub.2 sensor 10 will be described with reference to FIG. 16.
The usual O.sub.2 feedback operation is carried out up to time T.sub.20, and when an output from the O.sub.2 sensor 10 is rich (A/F ratio: 14.7 or less), an amount of fuel supplied to the engine 8 is decreased, whereas when the output from the O.sub.2 sensor 10 is lean (A/F ratio: 14.7 or more), an amount of fuel supplied to the engine 8 is increased, so that the amount of fuel is controlled to reverse the output from the O.sub.2 sensor 10.
When it is determined that the operating state of the engine 8 is in an O.sub.2 sensor malfunction determination mode (time T.sub.20), the ECU 9 decreases the amount of fuel supplied to the engine 8 to a first predetermined amount F.sub.1 for a first predetermined period of time (from time T.sub.20 to time T.sub.21) by controlling the injector 12 and thereafter increases the amount of fuel up to a second predetermined amount F.sub.2 for a second predetermined period of time (from time T.sub.21 to time T.sub.22).
When the O.sub.2 sensor normally operates, an output from the O.sub.2 sensor 10 decreases to a level V.sub.L1 at time T.sub.21 (i.e., when a lean period has finished) and thereafter reaches a preset determination level V.sub.TH or higher in a period of time t.sub.h1.
When the O.sub.2 sensor 10 is deteriorated, it is a general phenomenon that an output voltage thereof decreases or an output thereof delays in response. Therefore, with the deteriorated O.sub.2 sensor, an output from the O.sub.2 sensor decreases only to a level V.sub.L2 at the time T.sub.21 (when a lean period has finished) or a long period of time t.sub.h2 is required for the output to reach the determination level V.sub.TH or higher, and thus the deterioration of the O.sub.2 sensor can be determined.
Next, an operation for determining a malfunction of the fuel device will be described with reference to FIG. 17.
In the fuel device for carrying out O.sub.2 feedback control, an output from the O.sub.2 sensor 10 is made larger than 0.5 V when the detected A/F ratio is smaller than 14.7 (rich), whereas when the A/F ratio is greater than 14.7 (lean), the output is made smaller than 0.5 V. Thus, an amount of fuel to be supplied to the engine 8 is controlled to reverse an output from the O.sub.2 sensor 10 so as to set the A/F ratio to 14.7 (an optimum value in the performance of the engine operation or combustion) as described in the above determination of the malfunction of the O.sub.2 sensor 10. For example, an O.sub.2 feedback control compensation amount is realized by an integration compensation for gradually increasing or decreasing an amount of fuel with respect to a time factor, as shown in FIG. 17.
When the respective components of the fuel device usually operates normally (up to time T.sub.40), the feedback control compensation amount acts in the vicinity of 1.0. However, when an amount of fuel is compensated to achieve an A/F of 14.7 by carrying out the O.sub.2 feedback control at the time a component of the fuel device such as the injector 12 or the like is deteriorated, compensation is made to reduce a difference (an amount corresponding the deteriorated characteristics) between the characteristics of the deteriorated component and a corresponding normal component so that the amount of the feedback control compensation is shifted, as shown after time T.sub.41. Therefore, a degree of deterioration of the respective components of the fuel device can be detected from the shift amount of the feedback control compensation amount.
Since the conventional malfunction diagnosis device for the fuel-evaporated-gas processing device is arranged as described above, it has the following problems.
That is, when a malfunction of the O.sub.2 sensor is determined, an amount of fuel to be supplied to the engine 8 is forcibly decreased during the period from time T.sub.20 to Time T.sub.21, so that if this period coincides with, for example, the period from the time T.sub.11 to the time T.sub.12 (i.e., the period during which the fuel evaporated gas accumulated in the canister 3 is supplied to the engine 8 in a moment), as shown in FIG. 14, the compensations of fuel in both periods are canceled out and a mixed gas supplied to the engine 8 is not made rich. Thus, since the O.sub.2 feedback control compensation amount is made small regardless of whether the fuel-evaporated-gas processing device normally operates, there is a possibility that the fuel-evaporated-gas processing device is erroneously determined to be malfunctioning.
When the engine 8 is operating in such an unstable combustion state, misfiring may take place in the engine 8 so an uncombusted gas is emitted from the engine 8, making it impossible to correctly detect the A/F ratio. As a result, the O.sub.2 feedback control compensation amount often exhibits an erroneous behavior. Likewise, when the O.sub.2 sensor 10 itself malfunctions, the O.sub.2 feedback control compensation amount controlled by an output from the O.sub.2 sensor also exhibits an erroneous behavior. Further, when the fuel device malfunctions, the O.sub.2 feedback control compensation amount is greatly displaced from a center value which is contemplated to set the A/F ratio to the vicinity of 14.7, the reliability of the O.sub.2 feedback control compensation amount is also lowered in this case.
When the malfunction of the fuel-evaporated-gas processing device is determined by the A/F ratio detection system of FIG. 14, the O.sub.2 feedback control compensation amount is used as a parameter for the determination of a malfunction. Thus, when the engine 8 operates in such a state that the O.sub.2 feedback control compensation amount exhibits an erroneous behavior or an unreliably low value, it is difficult to correctly determine a malfunction of the fuel-evaporated-gas processing device.
After the beginning of the fuel-evaporated-gas processing device malfunction determination mode, the amount of the fuel evaporated gas filled in the canister 3 in a period (a period of time from time T.sub.0 to time T.sub.1 in FIG. 13 or a period of time from time T.sub.10 to time T.sub.11 in FIG. 14) during which the canister 3 is filled with the evaporated gas varies depending upon the operating state of the engine 8.
FIG. 18 shows the effect caused by the amount of the fuel evaporated gas generated in the fuel tank 1 when a malfunction of the fuel-evaporated-gas processing device is determined or checked. A normal state A shows the behavior of the pressure in the fuel tank 1 and the O.sub.2 feedback control compensation amount K.sub.FB when the fuel evaporated gas is not sufficiently absorbed by the canister 3. When there is a small amount of fuel evaporated gas in the fuel tank 1, however, even if the fuel-evaporated-gas passage is shut off, the pressure in the fuel tank 1 less increases, and even if a fuel evaporated gas stored thereafter is supplied to the engine 8, the A/F ratio is less affected by the fuel evaporated gas because the gas has a low concentration and thus a behavior shown by a normal state B in FIG. 18 is taken.
Even if the pressure in the fuel tank 1 changes around the time when the solenoid valve 6 is turned on and off, the A/F ratio of the fuel evaporated gas accumulated in the canister 3 is not always made rich depending upon the operating state of the engine 8 and thus there may be a case where the A/F ratio behaves as shown by the normal state B similarly to the aforesaid.
As a result, the behavior of the pressure in the fuel tank 1 and the O.sub.2 feedback control compensation amount K.sub.FB are near or like the behaviors taken in malfunction (broken line in FIG. 18) as compared with the case of the normal operation A, and since the pressure in the fuel tank 1 and the O.sub.2 feedback control compensation amount K.sub.FB less change, it may be difficult to set a malfunction determination value.
When the pressure in the fuel tank 1 and the O.sub.2 feedback control compensation amount K.sub.FB are changed by an error in the detection system or other factors, there is a possibility that a malfunction is erroneously determined in a worst case.